Increasingly, resources such as heavy crude oils, bitumen, tar sands, shale oils, and hydrocarbons derived from liquefying coal are being utilized as hydrocarbon sources due to decreasing availability of easily accessed light sweet crude oil reservoirs. These resources are disadvantaged relative to light sweet crude oils, containing significant amounts of heavy hydrocarbon fractions such as residue and asphaltenes, and often containing significant amounts of sulfur, nitrogen, metals, poly-aromatic hydrocarbon compounds, and/or naphthenic acids. The disadvantaged crudes typically require a considerable amount of upgrading, for example by cracking and by hydrotreating, in order to obtain more valuable hydrocarbon products. Cracking is generally effected by treating a crude or a heavy fraction of a crude at a temperature ranging from 375° C. to 500° C., optionally in the presence of a catalyst (catalytic cracking) and optionally in the presence of hydrogen (hydrocracking), and results in the decomposition of larger and heavier molecular weight constituents to smaller, lighter molecular weight compounds by cleavage of carbon-carbon linkages. Hydrotreating is generally effected by treating a crude or a fraction of a crude at a temperature ranging from 260° C. to 425° C. in the presence of hydrogen, and optionally in the presence of a catalyst, and results in reduction of sulfur, nitrogen, oxygen, and metals in the crude.
In conventional processes for upgrading disadvantaged heavy crude oil and bitumen feedstocks, the feedstocks are fractionated by distillation to separate the lightest distillate fractions, containing lower-boiling hydrocarbons, by atmospheric pressure distillation. Heavier fractions containing higher boiling fractions, called vacuum gas oils, are separated by vacuum distillation. The heaviest fraction called residue or pitch containing the highest, non-distillable, boiling hydrocarbons is produced as the bottoms fraction from the vacuum distillation. Lighter hydrocarbons may be recovered from the vacuum gas oils and residue fractions by fluid catalytic cracking or coking. Typically, vacuum gas oils are catalytically cracked in a Fluidized Catalytic Cracker (FCC) to produce lighter hydrocarbons, non-condensable hydrocarbon gases, and coke, where the lighter hydrocarbons may be blended with other distillate fractions to make fuel products. Residue may be cracked in a coker or hydrocracked in a residue hydrocracker to produce lighter hydrocarbon fractions, heavier residue fluid, non-condensable gases, and coke, where the lighter hydrocarbon fractions may be blended with other distillate fractions to make fuel products, and the residue fluid may be further cracked in a Residue Fluidized Catalytic Cracker (RFCC) to produce more light hydrocarbons. Typically, the separated fractions may be hydrotreated to reduce sulfur, nitrogen, and metals content of the fractions since heteroatoms and metals are undesirable in hydrocarbon products produced from the light distillate fractions, and act as hydrocracking catalyst poisons in the vacuum gas oil fraction and residue fraction.
Typically in a conventional process for upgrading a disadvantaged heavy crude feed or bitumen a maximum of about 70%-75% of the carbon content of the disadvantaged crude feed material is captured as non-residue, non-asphaltenic hydrocarbons that are liquid at standard temperature and pressure (STP—25° C., 0.101 MPa), the remainder of the carbon content being produced as gaseous hydrocarbons and carbonaceous solids such as coke. Furthermore, in a conventional process a large percentage of the sulfur and nitrogen are concentrated in high molecular weight heteroatomic hydrocarbons so that the sulfur and nitrogen become refractory, rendering removal of most or all of the sulfur or nitrogen from the hydrocarbon product difficult.
Alternatively, disadvantaged heavy crude oil and bitumen feedstocks may be hydrotreated and catalytically hydrocracked to produce an upgraded hydrocarbon product without initially separating the feedstock into fractions. Current “whole crude” heavy oil or bitumen feedstock upgrading processes also suffer from the production of excess coke and gas, and typically a maximum of about 70%-75% of the carbon content of the disadvantaged crude feed material is captured as non-residue, non-asphaltenic hydrocarbons that are liquid at STP. Current “whole crude” heavy oil or bitumen feedstock upgrading processes also create substantial quantities of refractory sulfur and nitrogen heteroatomic hydrocarbon compounds thereby rendering removal of most or all of the sulfur or nitrogen from the product difficult.
Formation of coke and refractory sulfur compounds is a particular problem in upgrading and refining heavy crudes and bitumen, whether as “whole crude” feedstocks or as fractions of a heavy crude or bitumen, that has limited the yield of desirable liquid hydrocarbons from such feedstocks. Cracking or hydrocracking, either thermal or catalytic, is required to obtain a high yield of hydrocarbons that are liquid at STP from a heavy crude or bitumen due to the large quantity of high molecular weight, heavy hydrocarbons such as residue and asphaltenes that are present in such feedstocks. Cracking hydrocarbons involves breaking bonds of the hydrocarbons, particularly carbon-carbon bonds, thereby forming two hydrocarbon radicals for each carbon-carbon bond that is cracked in a hydrocarbon molecule. Numerous reaction paths are available to the cracked hydrocarbon radicals, the most important being: 1) reaction with a hydrogen donor to form a stable hydrocarbon molecule that is smaller in terms of molecular weight than the original hydrocarbon from which it was derived; and 2) reaction with another hydrocarbon or another hydrocarbon radical to form a hydrocarbon molecule larger in terms of molecular weight than the cracked hydrocarbon radical—a process called annealation. The first reaction is desired, it produces hydrocarbons of lower molecular weight than the heavy hydrocarbons contained in the feedstock—and preferably produces naphtha, distillate, or gas oil hydrocarbons. The second reaction is undesired and leads to the production of coke and refractory sulfur containing heteroatomic hydrocarbons. Furthermore, the second reaction is autocatalytic since the cracked hydrocarbon radicals are reactive with the growing coke particles.
Hydrocarbon-containing feedstocks having a relatively high concentration of heavy hydrocarbon molecules therein are particularly susceptible to coking due to the presence of a large quantity of high molecular weight hydrocarbons in the feedstock with which cracked hydrocarbon radicals may combine to form proto-coke or coke. As a result, yields of non-residue, non-asphaltenic hydrocarbons that are liquid at STP from heavy crude oils and bitumen have been limited by coke formation induced by the cracking reaction itself.
Furthermore, sulfur tends to be concentrated in high molecular weight heteroatomic hydrocarbons in heavy crude oil and bitumen feedstocks. These molecules are also particularly susceptible to annealation due to the large quantity of large, high molecular weight sulfur-containing heteroatomic hydrocarbons in heavy oil and bitumen feedstocks. As a result, large quantities of refractory sulfur-containing heteroatomic hydrocarbons are formed in conventional cracking processes when utilizing a heavy crude oil or bitumen as a feedstock. A desirable characteristic of coking is that it tends to concentrate large aromatic ring structures, sulfur, nitrogen, and metals in the coke—leaving cracked, lighter hydrocarbon fragments of improved quality relative to the residue fraction of the feedstock—but this concentration effect is obtained at considerable expense in liquid product yield (at STP).
Numerous catalysts have been developed for use in processes for hydroprocessing disadvantaged hydrocarbon feedstocks, either as “whole crude” feeds or as heavy fractions of a heavy crude oil or bitumen, however, such catalysts have not eliminated problems associated with coking and production of refractory sulfur compounds, and catalyst activity may be significantly reduced over time by accumulation of coke on the catalyst. The formation of coke or sediment may be controlled by limiting the degree of conversion of residue range hydrocarbon in the feedstock to lighter hydrocarbons, which results in a yield loss with respect to the lighter, higher valued, hydrocarbon fractions. If not controlled, the formation of insoluble coke or sediment may lead to detrimental fouling of the residue hydroprocessing equipment and catalysts.
Conventional hydrocracking catalysts are generally selected to possess acidic properties that catalytically facilitate cracking by promoting the formation of cracked radical carbocation hydrocarbon species from hydrocarbons in the feedstock. Such catalysts typically include an acidic support, usually formed of alumina, silica, titania, or alumina-silica, on which a Group VIB metal or metal compound and/or a Group VIII metal or metal compound is deposited or interspersed to catalyze hydrogenation of the cracked radical hydrocarbon species. These catalysts likely promote the formation of coke and refractory sulfur compounds since they induce the formation of highly unstable and highly reactive carbocation radical hydrocarbon species without concomitantly hydrogenating the highly reactive carbocation radical hydrocarbon species as they are formed, thereby permitting a portion of the highly reactive radical hydrocarbon species to react with other hydrocarbons, heteroatomic hydrocarbons, or hydrocarbon radicals to form proto-coke or coke, and/or refractory heteroatomic sulfur-containing hydrocarbons.
Improved processes for processing heavy hydrocarbon-containing feedstocks to produce a lighter hydrocarbon-containing crude product are desirable, particularly in which coke formation is significantly reduced or eliminated, the yield of non-residue, non-asphaltenic hydrocarbons that are liquid at STP is increased so that at least 80%—and more preferably at least 90%—of the carbon content in the feed is captured in non-residue, non-asphaltenic hydrocarbons that are liquid at STP, and which contain little refractory sulfur compounds.